Hypertension afflicts about one-fourth of the world's adult population. It is a major risk factor for stroke and heart disease and is therefore a “silent killer”. Hypertension can be treated with lifestyle changes and medication. Medical therapy is associated with a 35-40% reduction in the risk of stroke and a 15-25% reduction in the risk of heart disease. Hence, hypertension management is an archetypical example of preventive, proactive healthcare. However, the detection of high blood pressure (BP) is often missed. An estimated 20% of people with hypertension in the US do not know they have it. Further, BP in known hypertensive patients is often uncontrolled. An estimated 53% of hypertensive patients in the US do not have their BP under control. Hypertension detection and control rates are much worse elsewhere, especially in low resource settings wherein personnel trained in BP measurement and the means for people to have their BP measured are lacking. Hypertension management is complicated by the well-known masked and white coat effects in the clinic and large BP variability amongst few measurements. In fact, ambulatory BP monitoring is now considered the gold standard for the diagnosis of high BP. Ubiquitous BP monitoring technology could improve hypertension detection by providing serial, out-of-clinic measurements in the mass population and could enhance hypertension control by providing continual feedback to the individual patient.
Several methods are available for measuring BP. However, none of these methods offers ubiquitous BP monitoring capabilities.
Catheterization is the gold standard method. This method measures a BP waveform by placing a strain gauge in fluid contact with blood. However, this method is invasive.
Auscultation is the standard clinical method. This method measures systolic BP (SP) and diastolic BP (DP) by occluding an artery with a cuff and detecting the Korotkoff sounds using a stethoscope and manometer during cuff deflation. The first sound indicates the initiation of turbulent flow and SP, while the fifth sound is silent and indicates the renewal of laminar flow and DP. The method is non-invasive but requires a skilled operator. Further, due to safety and ecological concerns, mercury manometers are being replaced with high maintenance aneroid manometers.
Oscillometry is the most popular non-invasive and automatic method. This method measures mean BP (MP), SP, and DP using an inflatable cuff with a sensor to record the pressure inside it. The recorded cuff pressure not only rises and falls with cuff inflation and deflation but also shows tiny oscillations indicating the pulsatile blood volume in the artery. The amplitude of these oscillations varies with the cuff pressure, as the arterial blood volume-transmural pressure relationship is nonlinear. Transmural pressure of an artery is defined as the internal pressure (i.e., BP) minus the external pressure (cuff pressure in this case). The BP values are estimated from the oscillogram (i.e., the oscillation amplitudes versus the cuff pressure) using an algorithm (e.g., fixed-ratios). However, automatic cuffs do not afford ubiquitous BP monitoring capabilities. That is, people in low resource settings may not have any access to such devices; others must go out of their way (e.g., to a pharmacy) to use these devices; and even people who own a device cannot carry and use them outside their homes.
Volume clamping is a non-invasive and automatic method used in research. This method measures a finger BP waveform by using a cuff with a photoplethysmography (PPG) sensor built-in to measure the blood volume. The blood volume at zero transmural pressure is estimated by slowly varying the cuff pressure. The cuff pressure is then continually varied to maintain this blood volume throughout the cardiac cycle via a fast servo-control system. The applied cuff pressure may thus equal BP. However, in addition to requiring a cuff, the method is prohibitively expensive.
Tonometry is another research method. This method measures a BP waveform by pressing a manometer-tipped probe on an artery. The probe must flatten or applanate the artery so that its wall tension is perpendicular to the probe. However, manual and automatic applanation have proven difficult. As a result, while the method should not require any calibration, the measured waveform has been routinely calibrated with a cuff in practice. Furthermore, the method is likewise costly.
As a result, cuff-less BP monitoring technology is being widely pursued. Much of these efforts are based on the principle of pulse transit time (PTT). PTT is the time delay for the pressure wave to travel between two arterial sites. An increase in BP causes the arteries to stiffen which, in turn, causes PTT to decline. So, PTT is often inversely correlated with BP in individual subjects. Further, PTT may be simply determined from the relative timing between proximal and distal arterial waveforms. Hence, PTT carries the advantage of possibly offering passive BP monitoring without using a cuff. However, this approach also has major disadvantages. Firstly, PTT not only changes with BP but also smooth muscle contraction (especially when measured in small arteries) and aging and disease (especially when measured in large arteries). Smooth muscle contraction occurs acutely and thus severely limits the accuracy of the approach, whereas aging and disease are longer processes that prevent PTT from being able to track chronic changes in BP such as the common development of isolated systolic hypertension due to large artery stiffening with aging. Secondly, the required calibration of PTT in units of msec to BP in units of mmHg must either be population-based and thus error-prone or involve periodic use of a BP cuff and thus not truly cuff-less.
In sum, hypertension is a major cardiovascular risk factor that is treatable, yet high BP detection and control rates are unacceptably low. Ubiquitous BP monitoring technology could improve hypertension management, but oscillometric and other available non-invasive BP measurement devices employ an inflatable cuff and therefore do not afford such monitoring capabilities. While the PTT approach could potentially permit cuff-less and passive BP monitoring, its accuracy will be limited due to confounding physiology and the need for calibration. Hence, there is a need in the art for a ubiquitous method for reliable, cuff-less measurement of BP.
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